Sometimes the refresh button works, sometimes not. If the refresh button doesn't work, I can sometimes get a thumbnail by right-click, select "reload" from the menu. If that doesn't work, last resort is to click on the dial to ensure it takes me to a working page. If it does, I then return to speed dial and right-click "reload" again, and then almost always the thumb pops into view.

Obviously, this needs refining so as to be more reliable. AND - if the link belongs to a moved page, the speed dial should show an "Error 404" thumbnail like old Opera used to.

Another thing to consider is if you are behind a proxy.
My Vivaldi lives behind a proxy and it does not refresh the speed dials if I have not loaded a page previously that made me enter my username and password for the proxy. It needs the user and pass to go online and refresh the speed dial, but it does not have it and it does not ask for it when you try to refresh the speed dial, so you must load a webpage first.